Lithographic manufacturing process, lithographic projection apparatus, and device manufactured thereby

ABSTRACT

A lithographic manufacturing process is disclosed in which first information of a lithographic transfer function of a first lithographic projection apparatus is obtained. The information is compared with second information of a reference lithographic transfer function (e.g. of a second lithographic projection apparatus). The difference between the first and second information is calculated. Then, the change of machine settings for the first lithographic projection apparatus, needed to minimize the difference, is calculated and applied to the first lithographic projection apparatus. In an exemplary application, a match between the first and second lithographic projection apparatus of any pitch-dependency of feature errors is improved.

RELATED APPLICATIONS

This application claims benefit of and is a divisional application ofU.S. patent application Ser. No. 10/114,309, filed Apr. 3, 2002 now U.S.Pat. No. 6,795,163, which claims priority from EP 01303201.6 filed Apr.4, 2001 and EP 02251207.3 filed Feb. 22, 2002, all three of thesedocuments being incorporated herein by reference.

FIELD

The invention relates generally to lithographic apparatus and moreparticularly to adjusting machine settings of a lithographic apparatusto reduce imaging errors.

BACKGROUND

In general, a lithographic manufacturing process according to oneembodiment of the invention comprises: providing a first lithographicprojection apparatus comprising a projection system for projecting apatterned beam onto a target portion of a substrate, providing asubstrate that is at least partially covered by a layer ofradiation-sensitive material, providing a projection beam of radiationusing a radiation system, using patterning structure to endow theprojection beam with a pattern in its cross-section, projecting thepatterned beam of radiation onto a target portion of the layer using theprojection system to obtain a projected image, and providing a secondlithographic projection apparatus for reference.

The term “patterning structure” as here employed should be broadlyinterpreted as referring to means that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate; theterm “light valve” can also be used in this context. Generally, the saidpattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such patterning structure include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired.    -   A programmable mirror array. One example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the said undiffracted light        can be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-adressable surface. An alternative embodiment of a        programmable mirror array employs a matrix arrangement of tiny        mirrors, each of which can be individually tilted about an axis        by applying a suitable localized electric field, or by employing        piezoelectric actuation means. Once again, the mirrors are        matrix-addressable, such that addressed mirrors will reflect an        incoming radiation beam in a different direction to unaddressed        mirrors; in this manner, the reflected beam is patterned        according to the addressing pattern of the matrix-adressable        mirrors. The required matrix addressing can be performed using        suitable electronic means. In both of the situations described        hereabove, the patterning structure can comprise one or more        programmable mirror arrays. More information on mirror arrays as        here referred to can be gleaned, for example, from U.S. Pat. No.        5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent        applications WO 98/38597 and WO 98/33096, which are incorporated        herein by reference. In the case of a programmable mirror array,        the said support structure may be embodied as a frame or table,        for example, which may be fixed or movable as required.    -   A programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.        For purposes of simplicity, the rest of this text may, at        certain locations, specifically direct itself to examples        involving a mask and mask table; however, the general principles        discussed in such instances should be seen in the broader        context of the patterning structure as hereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningstructure may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at one time; such an apparatus is commonly referredto as a wafer stepper. In an alternative apparatus—commonly referred toas a step-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTPatent Application No. WO 98/40791, incorporated herein by reference.

A circuit pattern corresponding to an individual layer of an ICgenerally comprises a plurality of device patterns and interconnectinglines. Device patterns may comprise structures of different spatialarrangement such as, for example, line-space patterns (“bar patterns”),capacitor and/or bit line contacts, DRAM isolation patterns, andtwin-hole patterns. Any such structure (of different spatialarrangement) is referred to hereinafter as a “feature”. Fabrication of acircuit pattern involves the control of space tolerances between devicesand interconnecting lines, between features, and between elements of afeature. In particular the control of space tolerance of the smallestspace between two lines permitted in the fabrication of the deviceand/or of the smallest width of a line is of importance. Said smallestspace and/or smallest width is referred to as the critical dimension(“CD”). A feature may comprise elements (such as, for example, bars)arranged in a spatially periodic manner. The length of the periodassociated with said spatially periodic arrangement is referred tohereinafter as the “pitch”. Hence, it is possible to identify a pitch ora (limited) range of pitches for such a (periodic) feature; accordingly,in this specification reference may be made to the pitch of a feature.Generally, a distinction is made between “dense” features and “isolated”features. In the context of the present invention, a dense feature is afeature where the width of a feature element is of the order of the CD,and where the pitch is of the order of two to six times the CD.Similarly, an isolated feature is a feature comprising elements of awidth of the order of the CD, and where the pitch is of the order of 6or more times the CD. Besides circuit patterns, a feature in the contextof at least one embodiment of the present invention may also relate to atest pattern for controlling a lithographic process step.

In lithography, a method known as CD-proximity matching is used toaddress a phenomenon known as the optical proximity effect. This effectis caused by the inherent difference in diffraction pattern for isolatedfeatures as compared to dense features. Generally, the optical proximityeffect leads to a difference in critical dimension (CD) when dense andmore isolated features are printed at the same time. A pitch dependenceof CD will be referred to hereinafter as “CD-pitch anomaly”. In thepresence of CD-pitch anomaly the printed CD depends on the pitch (theinverse of the spatial frequency) at which elements of the dimension ofthe CD are arranged in a feature.

CD-pitch anomaly also depends on the illumination setting used. An“illumination setting” or an “illumination mode” in the context of thepresent invention should be interpreted throughout this specificationand in the claims to comprise a setting of a preselected radiationintensity distribution in a pupil plane of the radiation system.Originally, so-called conventional illumination modes have been usedwhich have a disc-like intensity distribution of the illuminationradiation at the pupil of the radiation system. With the trend toimaging smaller features, the use of illumination settings providingannular or multi-pole intensity distributions in the pupil of theradiation system have become standard in order to improve the processwindow, i.e. exposure and focus latitude, for small features. However,CD-pitch anomaly becomes worse for off-axis illumination modes, such asannular illumination.

One solution to alleviate the occurrence of CD-pitch anomaly has been toapply Optical Proximity Correction (referred to hereinafter as “OPC”) bybiasing the different features on the reticle. For example, according toone form of biasing, the features are biased by making the lines of moreisolated features on the reticle somewhat thicker so that, in the imageon the substrate, they are printed with the same transverse dimension asthe lines of dense features. In another form of biasing, an endcorrection is applied so that the lines of isolated or dense featuresare printed with the correct length. However, at smaller pitches andwith off-axis illumination, the variation of the CD as a function ofpitch is more pronounced and more non-linearly related to pitch than atlarger pitches; consequently, more line biasing has to be applied atsmaller pitches and the biasing becomes more complicated. OPC isdiscussed, for example, in SPIE Vol. 4000, pages 1015 to 1023,“Automatic parallel optical proximity correction and verificationsystem”, Watanabe et al. As will be appreciated, advanced softwarealgorithms and very complex mask making is required for OPC. This hassignificantly increased costs of masks. Generally, in a high volumemanufacturing site, different lithographic projection apparatus are tobe used for a lithographic manufacturing process step involving OPC. Insuch a situation, matching of different lithographic projectionapparatus such as to reduce CD variations is normally done for oneselected feature type (for example, a dense or an isolated feature) byan adjustment of exposure energy at each of said different lithographicprojection apparatus save the apparatus used for reference. Such amatching of lithographic apparatus is described, for example, in C. Leeet al., “Reducing CD variation via statistically matching steppers”,Proceedings of the SPIE, Vol. 1261, 63–70 (1990) and in U.S. Pat. No.5,586,059, incorporated herein by reference. Since the matching ofprinted CD is substantially effectuated for one selected feature type,there is the problem that the matching of CD for features with pitchesother than the pitch of the feature selected for matching can be ratherpoor or even out of tolerance.

SUMMARY

According to at least one embodiment of the invention, a lithographicmanufacturing process comprises:

providing a first lithographic projection apparatus comprising aprojection system for projecting a patterned beam onto a target portionof a substrate;

providing a substrate that is at least partially covered by a layer ofradiation-sensitive material;

providing a projection beam of radiation using a radiation system;

using patterning structure to endow the projection beam with a patternin its cross-section;

projecting the patterned beam of radiation onto a target portion of thelayer using the projection system to obtain a projected image;

providing a second lithographic projection apparatus for reference;

obtaining first information on spatial-frequency dependency of a firstlithographic transfer function from the projected image in the radiationsensitive layer;

obtaining second information for reference on spatial-frequencydependency of a second lithographic transfer function using the secondlithographic projection apparatus;

calculating the difference between said first information and saidsecond information;

calculating a change of machine settings of the first lithographicprojection apparatus to apply to at least one of said machine settingsto minimize said difference; and

applying the calculated change of machine settings.

According to at least one embodiment of the invention there is provideda lithographic projection apparatus comprising:

a radiation system for providing a projection beam of radiation;

a support structure for supporting patterning structure, the patterningstructure serving to pattern the projection beam according to a desiredpattern;

a substrate table for holding a substrate;

a projection system for projecting the patterned beam onto a targetportion of the substrate;

machine settings applicable to at least one of exposure energy settingand illumination settings;

means for changing machine settings; and

a processor for calculating

the difference between a first information on a first lithographictransfer function and a second information on a second lithographictransfer function, and

a change of machine settings to be applied to at least one of saidmachine settings to minimize said difference.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5–20 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts and inwhich:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 shows plots of CD-pitch anomalies. Along the vertical axis theCD, as printed, is given, in nm, and along the horizontal axis the pitchis given, in nm.

FIG. 3 shows plots of the difference between CD-pitch anomalies and aCD-pitch anomaly used for reference. Along the vertical axis thedifference of CD, as printed, is given, in nm, and along the horizontalaxis the pitch is given, in nm.

FIG. 4 shows a plot of CD-pitch anomalies illustrating the effect of aspherical aberration induced non matching. Along the vertical axis theCD, as printed, is given, in nm, and along the horizontal axis the pitchis given, in nm.

FIG. 5 shows plots of the difference between a CD-pitch anomaly and aCD-pitch anomaly used for reference, and it shows the effect ofmachine-setting changes. Along the vertical axis the difference of CD,as printed, is given, in nm, and along the horizontal axis the pitch isgiven, in nm.

FIG. 6 shows a plot of CD-pitch anomaly illustrating the effect of usinga double exposure spherical aberration induced non matching. Along thevertical axis the CD, as printed, is given, in nm, and along thehorizontal axis the pitch is given, in nm.

DETAILED DESCRIPTION

It is well known that the imaging properties of a projection system canin principle be characterized by an Optical Transfer Function (OTF). Thephysical principles related to the concept of an OTF are discussed in,for example, “Image Science”, J. C. Dainty, Academic Press, 1974. Thisfunction describes the transfer of spatial frequency components by theprojection system. The pattern, as provided to the projection beam bythe patterning structure, features a certain spatial frequency spectrum.The product of this spectrum with the OTF yields the spatial frequencyspectrum of the image of said pattern. The OTF is a function of spatialfrequency, and generally the value of the OTF decreases for increasingspatial frequency. Consequently, an isolated feature is imageddifferently from a dense feature (the spatial frequency spectrum of anisolated feature contains substantially lower spatial frequencies ascompared to the spatial frequencies occurring in the spectrum of a densefeature). Similarly, a lithographic manufacturing process-step may becharacterized by a Lithographic Transfer Function, referred tohereinafter as an “LTF”. The LTF describes the transfer of spatialfrequencies from the pattern, as provided to the projection beam by thepatterning structure, to the pattern as printed. The term LithographicTransfer Function in the context of at least one embodiment of thepresent invention should be interpreted throughout this specificationand in the claims to include transfer functions which describe thetransfer of spatial frequencies from a phase and/or amplitude pattern ofelectromagnetic radiation (as provided by the patterning structure) tothe corresponding pattern as printed.

Clearly, an LTF is not unique. For example, procedures that thesubstrate may undergo prior and after exposure (as mentioned above), maycritically affect the spatial frequency content of a printed pattern,and hence, the spatial-frequency dependency of the LTF. Also, a processrun on a single lithographic projection apparatus may lead to differentLTF's. This is due to the fact that, given a specific pattern to beimaged, machine settings such as, for example, exposure dose andillumination settings may critically affect the pattern as printed, andhence may critically affect the spatial frequency content of the patternas printed. Inevitably, different lithographic projection apparatus,even of the same type, for a specific process step will have LTF's withdifferent spatial-frequency dependencies. This may be caused, forexample, by residual calibration errors of the machine settings, andresidual aberration errors of lenses of the respective differentlithographic projection apparatus. Also, lithographic projectionapparatus of different generations or types, although operated at thesame imaging specifications, may well yield different LTF's and hence,different printed patterns and/or CD's due to, for example, aberrationdifferences which are part of the nominal designs of the respective(first and second) projection systems. Since an LTF is, in the practiceof projection lithography, observable through a pitch dependence oflithographic feature errors, matching LTF's can be substantiallyrealized through matching the pitch dependency of lithographic featureerrors such as, for example, CD-pitch anomaly. Once a substantiallithographic-apparatus to lithographic-apparatus matching of LTF's hasbeen effectuated, an improved match of printed CD's for a plurality offeature types characterized by a corresponding plurality of (different)pitches is obtained. For simplicity, a lithographic-apparatus tolithographic-apparatus matching as described here may be referred tohereinafter as a “machine-to-machine” matching. An additional advantageof an improved machine-to-machine matching is that the effectiveness ofOPC measures provided to reticles is improved.

For application of a lithographic manufacturing process according to atleast one embodiment of the present invention it is not necessary toobtain complete first and second information on the spatial-frequencydependency of the first LTF and the second LTF respectively, completeinformation implying information for all spatial frequencies where theLTF's are non-vanishing. For matching purposes sufficient firstinformation on the first LTF can be the magnitude of any lithographicerror as occurring in, for example, two printed features, said twofeatures being characterized by two corresponding, different pitches.For simplicity, a lithographic error occurring in a printed feature maybe referred to hereinafter as a “feature error”. Generally, themagnitude of a feature error depends on the characteristic pitch forthat feature. Such errors can, for example, be measured or be calculatedusing commercially available lithography simulation software such asProlith™, Solid-C™ or LithoCruiser™. For example, given specific(critical) features to be imaged, given the aberration of the projectionsystem, given the data concerning the radiation sensitive layer on thesubstrate, and given the radiation beam properties such as radiationenergy and wavelength, predictions regarding the magnitude of featureerrors can be made with these simulation programs.

Similarly, the second information (for reference) can be measured orcalculated resulting in data for reference (“reference-feature errors”).The difference between the feature error and reference-feature errormagnitudes can then be determined for pitches of interest.

By introducing small variations of the machine settings which are knownto affect the feature error and by calculating the corresponding changesin the feature error for several pitches, coefficients quantifying therelationship between the feature error magnitude and saidmachine-setting changes can be established. These coefficients then alsoestablish the relation of the machine-setting changes with thedifference between the feature error and the reference-feature error forthe pitches of interest.

If the number of pitches of interest equals the number ofmachine-setting changes available for matching, above mentionedcalculations result in a set of equations where the number of equationsequals the number of unknown machine-setting changes (needed formatching). Such a set of equations can be solved in principle for zerodifference between the feature error and the reference feature error forany of the pitches. If the number of pitches exceeds the number ofmachine-setting changes to be used, one can instead minimize the featureerror difference at these pitches by means of, for example, a weightedleast squares minimization (a least square fit). Such a minimizationwill yield a set of machine-setting changes which optimizes in anobjective manner the match of the pitch dependent feature error. So, byapplying the set of machine-setting changes found this way, the matchbetween pitch-dependent errors as occurring between different machinesused for a specific lithographic manufacturing process can be improved.

It is an aspect of at least one embodiment of the current invention toin particular enable machine-to-machine CD-pitch anomaly-matching. It isknown that for dense features (small pitches) the exposure energy can beused to change the CD as it appears in a printed feature. For isolatedfeatures both the exposure energy and, for example, the illuminationsettings “σ-inner” and “σ-outer”, respectively defining the inner andouter radial extent (in relation to the numerical aperture of theprojection system) of an annular intensity distribution in the pupil ofthe radiation system, can be used to affect the printed CD. Given twomachine-setting changes available for use (exposure energy and, forexample, σ-outer), one can in principle choose two preferred pitches andobtain substantially CD-pitch anomaly-matching for critical featuresoccurring at these two pitches. The printed CD for isolated features isalso affected, for example, by the numerical aperture setting (alsoreferred to hereinafter as NA-setting) of the projection system, and bya displacement of the substrate in a direction substantiallyperpendicular to the exposed surface of the substrate. Such adisplacement will be referred to hereinafter as a change of focussetting of the substrate. Another possibility to affect printed CD forisolated features is, for example, by providing to the patternedprojection beam a preselected amount of even wave front aberrations.Said even wave front aberrations can, for example, comprise sphericalaberration. Projection systems comprising adjustable lens elementsgenerally enable a setting of a preselected amount of spherical wavefront aberration by adjusting the axial position (along the optical axisof the projection system) of one or more projection-systemlens-elements.

It is another aspect of at least one embodiment of the invention thatthe coefficients which quantify the relationship between themachine-setting changes and the pitch dependent error difference (whichis to be compensated by matching) can also be stored in a database assets or families of coefficients depending on the specific lithographicmanufacturing process step data such as the pattern type, theillumination setting, and the NA-setting.

According to a further aspect of at least one embodiment of theinvention, CD-pitch anomaly-matching is optimized exploiting thepossibility of double exposure. The pattern is split up into two subpatterns, each featuring a specific range of pitches. One range ofpitches contains pitches typical for dense features; the image formationof these dense features at the substrate comprises two-beam interference(i.e. the interference of, for example, a zeroth and a first orderdiffracted beam of radiation). For this range of pitches, the main causefor non ideal CD-pitch anomaly-matching is an exposure energy error.Matching can, for example, be achieved by an energy offset, whichresults in an off-set of the CD-pitch curve. The other range of pitchescontains pitches typical for isolated features. Here, the imageformation at the substrate comprises three-beam interference (involving,for example, a zeroth order, a plus-first and a minus-first orderdiffracted beam). Therefore more parameters such as, for example, focussetting, spherical wave front aberration, coherence σ-settings), andexposure energy play a role. Matching can be achieved by creating anoffset of the CD-pitch curve through an exposure dose offset, and bycreating a tilt (rotation) of the CD-pitch curve by means of, forexample, a σ-setting change. This method enables excellent CD-pitchanomaly-matching over a large range of pitches.

FIG. 1 schematically depicts a lithographic projection apparatusaccording to at least one embodiment of the present invention. Theapparatus comprises:

a radiation system Ex, IL, for supplying a projection beam PB ofradiation (e.g. UV radiation). In this particular case, the radiationsystem also comprises a radiation source LA;

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g. a reticle), and connected to first positioningmeans for accurately positioning the mask with respect to item PL;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to second positioning means for accurately positioning thesubstrate with respect to item PL;

a projection system (“lens”) PL (e.g. a quartz and/or CaF₂ lens systemor a catadioptric system comprising lens elements made from suchmaterials) for imaging an irradiated portion of the mask MA onto atarget portion C (e.g. comprising one or more dies) of the substrate W.As here depicted, the apparatus is of a transmissive type (i.e. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (with a reflective mask). Alternatively, the apparatusmay employ another kind of patterning structure, such as a programmablemirror array of a type as referred to above.

The source LA (e.g. a UV excimer laser) produces a beam of radiation.This beam is fed into an illumination system (illuminator) IL, eitherdirectly or after having traversed conditioning means, such as a beamexpander Ex, for example. The illuminator IL may comprise adjustingmeans AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionencompass at least both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g. after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step-and-scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed.

The depicted apparatus can be used in two different modes:

-   1. In step mode, the mask table MT is kept essentially stationary,    and an entire mask image is projected at one time (i.e. a single    “flash”) onto a target portion C. The substrate table WT is then    shifted in the x and/or y directions so that a different target    portion C can be irradiated by the beam PB;-   2. In scan mode, essentially the same scenario applies, except that    a given target portion C is not exposed in a single “flash”.    Instead, the mask table MT is movable in a given direction (the    so-called “scan direction”, e.g. the y direction) with a speed v, so    that the projection beam PB is caused to scan over a mask image;    concurrently, the substrate table WT is simultaneously moved in the    same or opposite direction at a speed V=Mv, in which M is the    magnification of the lens PL (typically, M=¼ or ⅕). In this manner,    a relatively large target portion C can be exposed, without having    to compromise on resolution.

According to at least one embodiment of the present invention, in FIG.2, graphs are shown which schematically indicate CD-pitch anomaly asoccurring in lithographic projection apparatus. Along the vertical andhorizontal axes the printed CD and the pitch are indicated,respectively. The graphs are representative for a lithographicmanufacturing process characterized by the following data: thewavelength λ of the radiation beam is 248 nm, the numerical aperture isNA=0.7, the σ-outer and σ-inner settings are, respectively, 0.8 and 0.55and the nominal CD is 130 nm. Graph 21 represents the CD-pitch anomalycharacteristic for a second lithographic projection apparatus. Thisapparatus will for simplicity be referred to, hereinafter, as the“reference tool”. Any other first lithographic projection apparatus thatis to be matched will be referred to hereinafter as “tool”. Graph 22represents the CD-pitch anomaly of a tool. A small change of exposuredose applied to the tool has the effect of translating graph 22 parallelto the vertical axis, as indicated by the arrow 23. In FIG. 3, theCD-pitch anomaly-match, in nm, and defined as the difference of the CDvalues of the graphs 21 and 22, is plotted as a function of pitch. Saidexposure dose adjustment has the same translational effect on the graphin FIG. 3 (as indicated by arrow 23) as it has on the graphs in FIG. 2.Therefore, an exposure dose adjustment can be used to shift the CD-pitchanomaly-match in FIG. 3 to the zero-nanometer level (i.e. the horizontalaxis in FIG. 3). To further improve the CD-pitch anomaly-match, theeffect of a change of, for example, the tool setting for σ-outer can beused. This effect is illustrated schematically in FIG. 2, and to goodapproximation consists of a rotation 24 about a point 25 of the part ofgraph 22 where the pitch is greater than a pitch Pr:Pr=λ/NA.  (1)The pitch Pr is indicated as pitch 26 in FIG. 2. Thus, a σ-outeradjustment results in CD-pitch anomaly graphs such as graphs 221 and 222in FIG. 2. The corresponding effect of a σ-outer adjustment on theCD-pitch anomaly-match is indicated in FIG. 3, and involves again saidrotation resulting in CD-pitch anomaly-match graphs 32 and 33, forexample. Clearly a combined σ-outer change and exposure dose changeresulting in a translation of graph 33 to the zero-nanometer level inFIG. 3 optimizes the CD-pitch anomaly-matching.

According to at least one embodiment of the present invention, saidrotation represented by the arrow 24 in FIG. 2 and FIG. 3 is effectuatedby a change of focus setting for displacing the substrate. Besides meansto provide and control accurately the focus setting of the substrate, alithographic projection apparatus may also comprise means to control andadjust spherical wave front aberration. Such means may compriseadjustable projection-system lens-elements. The effect of adjustingspherical aberration is substantially similar to the effect of a changeof focus setting, and is schematically represented by arrow 24 in FIG. 2and FIG. 3 as well. Hence, CD-pitch anomaly-matching may also comprise aprojection-system lens-element adjustment. Combinations of settingchanges, such as a σ-setting adjustment, a focus setting adjustment anda spherical aberration setting adjustment, can also be used forproviding a rotation 24 of a CD-pitch graph, and hence, for optimizingCD-pitch anomaly-matching.

According to at least one embodiment of the present invention, thepattern is split up into two sub patterns, one sub pattern substantiallycomprising features at pitches smaller than the pitch Pr, and the othersub pattern substantially comprising features at pitches greater thanthe pitch Pr. The advantage of said splitting up of the pattern inrelation to CD-pitch anomaly-matching (and exploiting the possibility ofdouble exposure) becomes clear when for example the effect of thepresence of residual higher order spherical aberration in the projectionsystem of the tool is considered. FIG. 4 shows CD-pitch anomaly graphsfor the lithographic manufacturing process described above, where thereference tool does not exhibit spherical aberration (graph 41), andwhere the projection system of the tool (which is to be matched)exhibits 0.05 waves of spherical aberration characterized by the Zernikecoefficient Z16 (graph 42). FIG. 5 shows a detailed plot of thecalculated, resulting CD-pitch anomaly-match, see graph 51. For densepitches (pitch<λ/NA), the match is within +1 and −1 nm, however, forisolated pitches (>λ/NA) a non matching occurs of up to +3 nm. Becauseof the use of double exposure, it is now possible to improve theCD-pitch anomaly-matching for pitches greater than λ/NA independent fromthe already obtained and sufficient CD-pitch anomaly-matching forpitches smaller than λ/NA. This is illustrated in FIG. 5, where acombined exposure dose (effect indicated by arrow 52) and σ-outeradjustment (effect indicated by arrow 53) are used to optimize theCD-pitch anomaly-matching for the sub pattern with pitches>Pr. Theresulting overall CD-pitch anomaly-matching for the double exposed imageis shown in FIG. 6: a match better than + and −1 nm is obtained for thecomplete range of pitches occurring in the pattern.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic apparatus comprising: a radiation system configured to provide a projection beam of radiation, said radiation system having an exposure energy setting and an illumination setting; a support structure configured to support patterning structure, the patterning structure serves to pattern the projection beam according to a desired pattern; a substrate table configured to hold a substrate; a projection system configured to project the patterned beam onto a target portion of the substrate; a memory device that provides information on spatial-frequency dependence of a lithographic transfer function of the lithographic apparatus and information on spatial-frequency dependence of a reference lithographic transfer function; and a processor configured to calculate (A) a difference between the information on spatial-frequency dependence of the lithographic transfer function of the lithographic apparatus and information on spatial-frequency dependence of the reference lithographic transfer function, and (B) a change of at least one of said exposure energy setting and said illumination setting to reduce said difference.
 2. The lithographic apparatus according to claim 1, wherein the information of the lithographic transfer function of the lithographic projection apparatus includes CD-pitch anomaly.
 3. The lithographic apparatus according to claim 1, wherein said patterning structure is configured to pattern the beam of radiation with the desired cross sectional pattern to obtain the patterned beam, wherein the desired cross sectional pattern comprises at least two sub-patterns, each of said at least two sub-patterns comprising features having a pitch within a respective range of pitches.
 4. The lithographic apparatus according to claim 3 wherein the respective ranges of pitches are selected such that pitch<1.5 λ/NA and 0.7 λ/NA<pitch, where λ/NA denotes a principal wavelength of the projection beam directed by a numerical aperture of the projection system.
 5. The lithographic apparatus according to claim 1, wherein the lithographic transfer function of the apparatus is based on an observed relation between critical dimension and pitch.
 6. The lithographic apparatus according to claim 5, wherein the lithographic transfer function of the apparatus includes a magnitude of lithographic errors that occur in two printed features, wherein the two printed features have different pitches.
 7. A lithographic apparatus comprising: a radiation system configured to provide a beam of radiation; a support structure configured to support patterning structure, the patterning structure serves to pattern the beam of radiation according to a desired pattern; a substrate table configured to hold a substrate; a projection system configured to project the patterned beam onto a target portion of the substrate; a memory device that provides information on spatial-frequency dependence of a lithographic transfer function of the lithographic apparatus and information on spatial-frequency dependence of a reference lithographic transfer function; and a processor configured to calculate (A) a difference between the information on spatial-frequency dependence of the lithographic transfer function of the lithographic apparatus and information on spatial-frequency dependence of the reference lithographic transfer function and (B) a change of at least one machine setting of the lithographic apparatus to reduce said difference.
 8. The lithographic apparatus according to claim 7, wherein the change of at least one machine setting includes (i) a change of exposure dose, (ii) a change of illumination setting, (iii) a change of numerical aperture of the projection system, (iv) a change of focus setting of the substrate, (v) a change of position of at least one lens element of the projection system or (vi) any combination of (i)–(v).
 9. The lithographic apparatus according to claim 7, wherein said processor is configured to calculate the change of at least one machine setting based on a plurality of coefficients indicating quantitative relationships between (A) at least one difference between information of the lithographic transfer function of the lithographic apparatus and information of the reference lithographic transfer function and (B) at least one machine setting of the first lithographic projection apparatus.
 10. The lithographic apparatus according to claim 9, wherein said processor is configured to select the plurality of coefficients from among a larger plurality of coefficients, based on a group consisting of (i) an illumination setting of the lithographic apparatus, (ii) a setting of the projection system, (iii) a feature of the desired pattern or (iv) any combination of (i)–(iii).
 11. The lithographic apparatus according to claim 7, wherein the lithographic transfer function of the apparatus is based on a observed relation between critical dimension and pitch.
 12. A lithographic apparatus comprising: means for providing a beam of radiation; means for endowing the beam of radiation with a pattern in its cross-section; means for projecting the patterned beam onto a target portion of a layer of radiation-sensitive material; means for obtaining information on spatial-frequency dependence of a lithographic transfer function of the apparatus; and means for calculating a change of machine settings based on a relation between (A) the information on spatial-frequency dependence of the lithographic transfer function of the apparatus and (B) information on spatial-frequency dependence of a reference lithographic transfer function.
 13. The lithographic apparatus according to claim 12, wherein said means for calculating the change of machine settings is configured to calculate the change of machine settings based on a difference between the information on spatial-frequency dependence of the lithographic transfer function of the apparatus and the information on spatial-frequency dependence of the reference lithographic transfer function.
 14. The lithographic apparatus according to claim 13, wherein said means for calculating a change of machine settings is configured to calculate the change of machine settings according to a minimization of the difference.
 15. The lithographic apparatus according to claim 12, said apparatus further comprising means for applying the change in machine settings.
 16. The lithographic apparatus according to claim 12, wherein the lithographic transfer function of the apparatus is based on a observed relation between critical dimension and pitch.
 17. A lithographic apparatus comprising: means for providing a beam of radiation, said means having an exposure energy setting and an illumination setting; means for supporting patterning structure, the patterning structure serving to pattern the beam of radiation according to a desired pattern; means for projecting the patterned beam onto a target portion of a substrate; memory means for providing information on spatial-frequency dependence of a lithographic transfer function of the lithographic apparatus and information on spatial-frequency dependence of a reference lithographic transfer function; and means for calculating, based on a relation between the information on spatial-frequency dependence of lithographic transfer function of the apparatus and information on spatial-frequency dependence of the reference lithographic transfer function, a change to be applied to at least one of the exposure energy setting and the illumination setting.
 18. The lithographic apparatus according to claim 17, wherein said means for calculating a change is configured to calculate the change based on a difference between information of the lithographic transfer function of the apparatus and information of the reference lithographic transfer function.
 19. The lithographic apparatus according to claim 18, wherein said means for calculating a change is configured to calculate the change according to a minimization of the difference.
 20. The lithographic apparatus according to claim 17, wherein the lithographic transfer function of the apparatus is based on a observed relation between critical dimension and pitch. 